A question was asked about the differing results from these two tests when deciding upon which unit to buy:
The boiler AFUE test is accomplished during a steady-state test firing of at least 30 minutes, using 120º input water and 140º output water, shutting down for 55 minutes, allowing the temperature to decay and restarting for 5 minutes, and recording the reaction to restarting. It is the standard DOE test and the AFUE is its measure of efficiency. The only heat loss is considered to be the heat that is lost up the stack. Pipe loss in the conditioned area of the home is still considered heat gain. All manufacturers test to the AFUE standard. This determines the input BTU, output BTU and the efficiency of the boiler.
The GAMA test uses the input BTU and output BTU determined by the AFUE test and divides the output BTU by 1.15 to account for pipe heat loss, which is not considered in the AFUE test, and results in a lower net output.
Too closely following the AFUE results when sizing a boiler can result in an undersized boiler, especially if there is an extended period of non-typical low temperatures.
Too closely following the GAMA results can potentially oversize the boiler.
A boiler with multiple firing rates can be used to overcome the problem of sizing too closely to a heat loss calculation or for a potential addition to the home.